A secondary battery is a battery that can repeatedly perform a charging operation of changing chemical energy into electric energy, and a discharging operation which is a reverse operation to the charging operation. Examples of the secondary battery include a nickel cadmium (Ni—Cd) battery, a nickel hydrogen (Ni-MH) battery, a lithium metal battery, a lithium ion (Ni-Ion) battery, and a lithium-ion polymer (Li-Ion Polymer) battery (hereinafter, abbreviated to LIPB).
A secondary battery is made up of an anode, a cathode, an electrolyte, and a separator, and it stores and generates electricity using a voltage difference between the cathode and the anode. The discharging operation involves transferring electrons from the cathode, having a higher voltage, to the anode, having a lower voltage, while generating electricity corresponding to a voltage difference between the cathode and the anode. Conversely, the charging operation is an operation of transferring electrons from the anode to the cathode. During the charging operation, an anode material receives electrons and lithium ions, and thus returns to a metal oxide state. That is, in a secondary battery, a charging current flows when metal atoms move from the anode to the cathode through the separator during the charging operation, and a discharging current flows when metal atoms move from the cathode to the anode during the discharging operation.
Lithium secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the kind of electrolyte. A battery that uses a liquid electrolyte is referred to as a lithium ion battery and a battery that uses a polymer electrolyte is referred to as a lithium polymer battery. An outer covering of a lithium secondary battery may take various forms. For example, it may have a cylindrical shape, a prismatic shape, or a pouch shape. An electrode assembly in which an anode plate, a separator, and a cathode plate are stacked or wound in that order is disposed inside the outer covering of a lithium secondary battery.
Secondary batteries have been used in various fields such as IT products, vehicles, and energy storage and currently attract attention as a promising energy source. Secondary batteries for IT products are required to ensure long operation time and have a small and lightweight body. Meanwhile, secondary batteries for use in vehicles are required to be durable and have high output power and stability (i.e. being free from risk of explosion). On the other hand, secondary batteries used for energy storage may not be required to have highly advanced features. Research and development of lithium secondary batteries began in early 1970, and lithium ion batteries which use carbon as a cathode material instead of lithium were put into practical use in 1990. The lithium ion batteries having a life cycle of 500 times and featuring a short charging time of about 1 to 2 hours have showed the highest increase in sales among secondary batteries. Lithium ion batteries are advantageous in reducing a weight of products that use a battery because they are 30 to 40% lighter than nickel-hydrogen batteries. In addition, lithium secondary batteries have the highest unit cell voltage (for example, a voltage in the range from 3.0 V to 3.7 V) among various kinds of secondary batteries and have high energy density. Therefore, lithium secondary batteries are suitable for use in mobile devices.
Secondary batteries are generally used in the form of a battery module in which one or more unit cells is stacked. In this case, when there exists a gap between stacked unit cells due to deterioration in adhesion between the stacked unit cells or swelling of the stacked unit cells, electrical sparks are likely to occur between the stacked unit cells, which may result in ignition or explosion. Therefore, reliability in electrical properties of stacked secondary battery modules or stability in operation of devices that use the secondary battery modules may be deteriorated.